1. Technical Field
The invention is related to plasma reactors of the type having a pair of electrodes for capacitively coupling radio frequency (RF) power to a plasma and in particular to improvements for permitting use of high RF frequencies and high RF power.
2. Background Art
In plasma etching of a thin film (such as a metal layer) on a semiconductor wafer, the wafer is placed within a vacuum chamber, the chamber is filled with reactive gases and RF power is coupled to the gases within the chamber to ignite and maintain a plasma therein. The plasma etches the thin film. The RF coupling to the plasma may be either capacitive or inductive. Inductive coupling is typically carried out with an inductive coil wound around the chamber and connected to an RF source. Capacitive coupling is typically carried out by a pair of electrodes, namely a cathode at the bottom of the chamber serving as a wafer pedestal on which the wafer is placed and an anode at the top of the chamber. The anode can serve as a gas distribution plate over the wafer, in which case the anode has many orifices facing the wafer through which the reactive gases are introduced into the chamber in a uniform manner. The gas distribution plate can be either grounded or connected to RF power. If the gas distribution plate/anode is grounded, the chamber wall may also be grounded so that the entire chamber wall as well as the gas distribution plate functions as an anode. In this case, the effective area of the anode is much greater than the area of the cathode/wafer pedestal.
One problem is that in etching an oxide thin film (such as silicon dioxide) from the wafer, material derived from the oxide (such as silicon) is deposited on the surfaces within the chamber exposed to the plasma. Such deposition of contaminants will accumulate most rapidly on grounded surfaces, because such surfaces are subject to the least ion bombardment from the plasma. Thus, one way of minimizing accumulation of such contaminant deposition is to apply RF power to such surfaces. Therefore, in order to keep the gas distribution plate/anode clean, and thereby suppress particle contamination, it is desirable to apply RF power to the gas distribution plate/anode, so that RF power is applied to both the anode and cathode.
One way of applying RF power to the cathode and anode might be to connect separate RF sources to each electrode. One disadvantage of this approach is that the chamber side wall would serve as the common ground for both RF sources so that a large portion of the plasma ions, rather than being confined between the two electrodes, instead would drift toward the chamber wall and away from the wafer. The problem is that such off-normal ion directionality (relative to the plane of the wafer) results in severe non-uniform oxide etch patterns and lower oxide etch rates. This problem arises because the plasma-assisted oxide etch process is largely reactive ion-driven. One solution to this problem is to power both electrodes (anode and cathode) from the opposing terminals of the same RF source, as disclosed in U.S. Pat. No. 4,626,312 to Tracy and in U.S. Pat. No. 4,871,421 to Ogle and Yin. A transformer can couple RF power to the anode and cathode, the RF source being connected across the primary winding and the anode and cathode being connected to opposite ends of the secondary winding.
There are three problems with the prior art exemplified by the patent to Tracy and the patent to Ogle and Yin. First, only low RF frequencies, on the order of 4 Mhz, may be employed. Otherwise, the power loss through the transformer would be prohibitive. A second problem is that the cathode-to-anode power ratio is not adjustable. This is a problem because in plasma etching of a dielectric or oxide thin film, a different cathode-to-anode power ratio is required, depending upon whether the process is (1) a reactive ion-driven plasma etch process or (2) a neutral species-driven ion-assisted plasma etch process (for example). In the patent to Tracy, the power ratio is floating and therefore not controllable while in the patent to Ogle and Yin the power ratio is permanently fixed at 50% (by the center ground tap on the transformer secondary winding) and therefore not adjustable. A third problem is that the ion energy is too high, resulting in unacceptably high incidence of device damage on the wafer. The high ion energy arises because the low RF frequency (which must be used to avoid power loss at the transformer) more readily accelerates the heavier plasma ions. Since the cathode/anode power ratio is not adjustable, ion energy at the cathode cannot be reduced to avoid device damage.